Lucas Matthysse TKO’d Lamont Peterson in the third round in April. (Getty Images)

A September mega-fight between undefeated boxer Floyd Mayweather and rising star Saul “Canelo” Alvarez in Las Vegas got even bigger on Thursday. A title-unifying fight between Philadelphia’s Danny Garcia and Argentina’s Lucas Matthysse was added as the top undercard, the fight’s promoter, Golden Boy, announced.

The Garcia-Matthysse fight, with the WBA and WBC titles on the line, could have wide-ranging implications at 140 pounds. The two boxers, along with Denver’s Mike Alvarado, make up the three best in the junior welterweight division.

“Matthysse is a good fighter and has a big punch, but I’m a talented fighter with what it takes to be a champion and stay that way,” said Garcia, who owns WBA and WBC titles in the 140-pound division.

Garcia (26-0) is coming off a unanimous decision victory over Zab Judah in April. Matthysse (34-2-0) has earned a reputation as one of the hardest-punching fighters in boxing. He last knocked out Lamont Peterson in May.

“This is the fight that the entire boxing world, especially my country Argentina, and I wanted,” Matthysse said.

The Sept. 14 card on Mexican Independence weekend has the potential to threaten boxing’s record for most watched pay-per-view fight. Mayweather’s split-decision victory over Oscar De La Hoya in 2007 drew more than 2.5 million PPV buys, still the record.

Mayweather (44-0) will fight for a second time in a calendar year for the first time since 2007 when he faces the 23-year-old Alvarez (42-0-1) from Mexico.